1. Field of the Invention
The present invention relates to a phase-contrast X-ray imaging system, in detail relates to an X-ray imaging system which provides extremely high sensitivity compared with that provided by a conventional type X-ray imaging method depending upon absorption contrast. The phase-contrast X-ray imaging system according to the present invention is suitable for observing biological soft tissues and others whose X-ray absorbing power is small and provides a relatively wide view field, enabling medical diagnosis.
2. Description of the Related Art
All currently realized X-ray clinical imaging systems obtain an image contrast based upon the quantity of absorbed X-rays. Because heavier elements absorb more X-rays, an object containing more heavy elements creates clearer X-ray shadow. However, an object made of light elements (soft tissues, etc.) which does not absorb X-rays so much is too transparent for X-rays to create a sufficient contrast. When such an object is needed to be investigated with conventional clinical X-ray imaging systems, a contrast medium containing heavy elements is injected into a soft tissue although such an injection procedure is not always possible. In the case of an X-ray imaging system for diagnosing breast cancer (mammography), one compromises to use relatively low-energy X-rays to increase the sensitivity to soft tissues (in this case, breast cancer), because it is difficult to emphasize breast cancer with a contrast medium. Using low-energy X-rays bases on the fact that X-ray absorption coefficient is inversely proportional to the third power of X-ray energy and that comparatively clear contrast appears. However, as the dose of X-rays is also inversely proportional to the third power of X-ray energy, one has to compromise the increase in the dose of X-rays caused by using low-energy X-rays. Nevertheless, quality of obtained images is not always sufficient for medical diagnosis.
On the other hand, there is an imaging method for obtaining a contrast from X-ray phase shift instead of X-ray absorption. As the interaction cross section of the X-ray phase shift is approximately a thousand times as large as the interaction cross section of X-ray absorption for light elements, observation is possible with sensitivity several hundreds times higher than the absorption-contrast method. This suggests that weakly X-ray absorbing objects can be observed without using special contrast media, and the sensitivity of phase-contrast X-ray imaging was demonstrated experimentally using an X-ray interferometer. However, there is no X-ray interferometer whose size is sufficiently large for a clinical use. Several-millimeter view field has ever been realized in Phase-contrast X-ray radiography (A. Momose, et al., Med. Phys., 22, 375-380 (1995)) and Phase-contrast computed tomography (A. Momose, et al., Rev. Sci. Instrum. 66, 1434-1436 (1995), the U.S. patent application Ser. No. 5,173,928).
Currently known typical X-ray interferometers are monolithically cut out from an ingot of single crystal of silicon or others as shown in FIG. 1. Three wafers 1 to 3 are formed in parallel each other with the same gap between them. When an incident X-ray beam 4 satisfies the Bragg diffraction condition for lattice planes 5, the incident X-ray beam 4 is separated into two beams 6a and 7a. The beam 6a is similarly separated into two beams 6b and 6c and the beam 7a into two beams 7b and 7c by the second wafer 2. The beams 6b and 7b are mixed by the third wafer 3 and interfere each other. That is, the three wafers 1 to 3 function as X-ray half mirrors, and two paths of interferring beams are formed. When an object 8 is inserted in one of the paths of the interfering beams, for example the beam 6b, the phase of the beam shifts and an interference pattern is formed in X-ray beams 6d and 7d outgoing from the third wafer 3. As the size of a view field is equivalent to the thickness of X-ray beams through the interferometer and two beam paths 6a to 6b and 7a to 7b are required to be spatially separated completely, a large interferometer is needed to provide a large view field. Estimating from the size of silicon ingots currently available, the maximum view field is approximately 2 cm.times.2 cm.
An X-ray interferometer comprising separated two crystal blocks which each have two X-ray half mirrors was reported by P. Becker and U. Bonse in J. Appl. Cryst. 7, 593-598 (1974). They studied a basic function of the separated X-ray interferometer and reported interference patterns with a size of 4 mm.times.8 mm. However, no remarkable development has not been reported to provide a large view field.